1. Field of the Invention
The present invention relates to an error correction decoder and decoding method thereof for allowing information to be transmitted and received reliably in a high speed wireless communication system, and more particularly, to a bandwidth-efficient concatenated trellis-coded modulation (TCM) decoder which is realized by combining turbo codes having an advantage of coping effectively with a fading channel with TCM having an advantage of bandwidth efficiency, and decoding method thereof.
2. Description of the Related Art
As the information society has evolved recently, a higher transmission rate of information is required. Accordingly, approaches for efficiently transmitting a large amount of information using a given bandwidth have been lively studied. In addition, considering mobility significant, relevant studies have been concentrated on approaches for covering areas handled by existing wire communication with wireless communication. To meet these requirements, a high speed wireless communication system has been raised as an important matter.
A high speed wireless communication system is required to reliably transmit a possibly large amount of information at high speed using a given bandwidth. To realize such reliable high speed wireless communication, it should be possible to reliably communicate information with a small amount of redundancy added to information to be transmitted. In addition, to realize wireless communication having reliability and ensuring mobility, a communication system should be designed to cope with InterSymbol Interference (ISI) occurring in a wireless communication system.
Generally, the gain of a channel code is obtained from bandwidth expansion. In other words, the-reliability of information to be transmitted is ensured using redundancy. Although error correction ability increases in proportional to the amount of such overhead bits used, a data transmission rate decreases, thereby reducing bandwidth efficiency. In other words, a decrease in a data transmission rate or an increase in transmission power should be paid for securing the reliability. Generally, three variables, i.e., power, a bandwidth and an error probability, are in a trade-off relation so that one of them should be sacrificed in order to obtain the other factors.
To overcome this problem, a technique in which modulation referred to as TCM allowing coding gain to be obtained without expanding a bandwidth is combined with a coding technique has been introduced by Ungerboeck. The TCM mainly aims at obtaining better coding gain without an increase in a bandwidth than a case where encoding is not performed. This seems to violate the trade-off relation among power, a bandwidth and an error probability, but this may be considered effected by the trade-off relation between coding gain obtained by TCM and decoder complexity.
TCM is achieved by combining a multilevel/phase modulation signal set with a trellis coding scheme. In TCM, a signal set may be considered as a single redundancy. In other words, predetermined 2m information bits of a signal set is increased to 2m+1, and then the signal set is encoded, so that coding gain is obtained but decoder complexity increases. According to TCM, information bits can be transmitted at a high transmission rate through an additional white Gaussian noise (AWGN) channel. Therefore, bandwidth efficiency can be increased. However, TCM has a disadvantage in that reliability considerably drops in a fading channel because it is very sensitive to ISI. Accordingly, TCM has been disregarded in the field of wireless communication even if it has the above advantage and has maintained its existence only in the field of MODEM of wire communication.
A turbo code method was introduced as concatenated coding and iterative decoding in the channel code field. The turbo code method announced by Berrou in 1993 is a powerful error correction coding method which realizes a low signal to noise ratio (SNR) and minimizes the probability of an error occurring due to signal distortions such as fading, ISI and AWGN. The turbo code method in which a code rate is xc2xd, a turbo coder generating formula is G1=37 and G2=21, and the size of an interleaver is 256xc3x97256 has excellent error correction performance such that Eb/N0 for a bit error probability Pe=10xe2x88x925 when the number of iterations of decoding is 18 in an AWGN channel is 0.7 dB. Due to the excellent error correction ability and steadiness against fading and interference, turbo codes recently tend to be used as channel codes in the field of wire and wireless communication.
However, the turbo codes usually have a code rate of at most xc2xd due to their structure. In other words, overhead bits as many as or more than information bits to be transmitted should be transmitted together with the information bits, so that a data transmission rate decreases, and bandwidth efficiency is low. However, the turbo codes have a steady characteristic to a fading channel so that fading can be overcome in a high speed wireless communication channel by using turbo codes having excellent error correction performance.
To solve the above problems, it is an object of the present invention to provide a bandwidth-efficient concatenated trellis-coded modulation (TCM) decoder for combining turbo codes and TCM to overcome the low code rate of the turbo codes and the degradation of TCM with respect to a fading channel, and a decoding method thereof.
Accordingly, to achieve the above object of the invention, in one aspect, there is provided a bandwidth-efficient concatenated TCM encoder including a basic TCM encoding unit for receiving a predetermined m-bit data sequence, recursively generating a parity bit per data, and mapping a (m+1)-bit code word sequence including the parity bit into M-ary symbols; at least one additional TCM encoding unit for receiving the predetermined m-bit data sequence, pairwise interleaving the m-bit data sequence in a predetermined manner, recursively generating a parity bit per data of the pairwise interleaved data sequence, mapping a (m+1)-bit code word sequence including the parity bit into M-ary symbols, and pairwise de-interleaving the M-ary symbols in a manner corresponding to the predetermined manner; and a switch for puncturing the outputs of the basic TCM encoding unit and the at least one additional TCM encoding unit.
In another aspect, there is provided a bandwidth-efficient concatenated TCM decoder including a demultiplexer for distributing a predetermined number of symbols that have been encoded by a bandwidth-efficient concatenated TCM encoder and received through a channel, first through N-th TCM decoding units (N is an integer of 2 or larger) for each receiving a current priori probability and symbols distributed by the demultiplexer, generating a detection value and an extrinsic value of the detection value and multiplying the detection value by the extrinsic value to generate a new priori probability, and a data detector for detecting final data from an output of the N-th TCM decoding unit after the operation performed by the first through N-th TCM decoding units is repeated a predetermined number of times. The first through N-th TCM decoding units are connected in the form of a loop, and each of them generates a priori probability and transfers it to a next TCM decoding unit as the current priori probability.
In yet another aspect, there is provided a bandwidth-efficient concatenated TCM encoding method including the steps of (a) for receiving a predetermined m-bit data sequence, recursively generating a parity bit per data, and mapping a (m+1)-bit code word sequence including the parity bit into M-ary symbols; (b) receiving the predetermined m-bit data sequence, pairwise interleaving the m-bit data sequence in a predetermined manner, recursively generating a parity bit per data of the pairwise interleaved data sequence, mapping a (m+1)-bit code word sequence including the parity bit into M-ary symbols, and pairwise de-interleaving the M-ary symbols in a manner corresponding to the predetermined manner; and (c) puncturing and transmitting the symbols generated in the steps (a) and (b).
In still yet another aspect, there is provided a bandwidth-efficient concatenated TCM decoding method including the steps of (a) generating an initial priori probability from a predetermined number of symbols received; (b) multiplying a detection value and an extrinsic value of the detection value together which are generated using some of the predetermined number of symbols received and the initial priori probability, to generate a new priori probability; (c) setting the new priori probability as a current priori probability that has been pairwise interleaved in a predetermined manner and multiplying a detection value and an extrinsic value of the detection value together which are generated using some of the predetermined number of symbols received by the current priori probability, to generate another new priori probability; and (d) replacing the initial priori probability in the step (b) with the priori probability generated in the step (c) and repeating the steps (b) and (c) a predetermined number of times and then detecting final data.